1. Field of the Invention
The invention relates generally to control of a rotating machine, and more particularly, to sensorless control of a permanent magnet rotating machine.
2. Description of Related Art
Permanent magnet machines, such as brushless permanent magnet motors, are well known. For example, a permanent magnet motor typically includes a stator with phase windings therein and a rotor with permanent magnets arranged to rotate relative to the stator. Position sensing devices are provided to indicate the rotor position with respect to the stator. The phase winding energization is generally controlled by solid state switches that are responsive to the rotor position indications provided by the position sensor to energize the windings in the proper commutated sequence. The phase windings may be energized by sine wave excitation or by angle controlled square wave excitation, for example.
The phase winding excitation depends on a scheme for synchronizing electromagnetic fields with the rotor permanent magnets. This includes tracking the angular position of the rotor, which carries the permanent magnets, via the position sensors. The rotor position sensors, however, can be quite expensive. Depending on the level of motor control required to perform a particular function, this cost can represent an appreciable portion of the motor""s cost. In addition, sensors take up space within a motor housing as does their associated wiring. Still further, sensors can fail, resulting in a significant degradation of the motor""s performance or even causing the motor to become inoperative.
To eliminate the need for position sensors, various xe2x80x9csensorlessxe2x80x9d motor constructions have been proposed. Unfortunately, the known sensorless schemes for PM motors have been largely unsatisfactory. For example prior art sensorless PM machines tend to be susceptible to noise problems, and performance tends to vary with rotor speed. Moreover, typical known sensorless PM machines only function for sine wave or 120 degree square wave excitation of the machine.
Thus, a need exists for a high performance sensorless control system for rotating PM machines that addresses the shortcomings of the prior art.
In one aspect of the present invention, a permanent magnet rotating machine, such as a permanent magnet AC motor, and a control system for such a machine includes an estimator that calculates an estimate of the machine""s rotor speed. The estimator receives energization feed back from the machine in a first rotating reference frame and outputs the rotor speed estimate in a second rotating reference frame. A controller receives a rotor speed demand and the rotor speed estimate, and outputs control signals for controlling energization of the machine in response to the rotor speed demand and rotor speed estimate signals.
In other aspects of the invention, a method of controlling a permanent magnet rotating machine includes receiving a rotor speed demand and generating a rotor speed command in response to the speed demand. A first rotating reference frame rotating at the commanded rotor speed is generated, and phase winding energization feed back is transformed from a stationary reference frame to the first rotating reference frame. The actual speed of the machine""s rotor is estimated, and a second rotating reference frame rotating at the estimated actual rotor speed is generated. The energization feed back is transformed from the first rotating reference frame to the second rotating reference frame, and phase energization commands are calculated. The phase energization commands are transferred from the second rotating reference frame to the stationary reference frame for application to the machine phase windings.
The use of the two rotating reference frames improves performance of the control system across the machine""s speed range. In exemplary implementations, a rotor position command is calculated based on the rotor speed command and the first rotating reference frame is generated based thereon. The actual rotor position may be estimated based on estimates of the magnet induced flux linkage.